The present invention relates to a process for the production of a syngas feed with a variable H2/CO ratio, which may be used in syngas conversion processes.
The conversion of natural gas assets into more valuable chemicals, including combustible liquid fuels, is desired to more effectively utilize these natural gas assets. The conversion of natural gas to more valuable chemical products generally involves syngas generation. Syngas generation involves converting natural gas, which is mostly methane, to synthesis or syngas gas, which is a mixture of carbon monoxide and hydrogen. Syngas may be used as a feedstock for producing a wide range of chemicals, including combustible liquid fuels, methanol, acetic acid, dimethyl ether, oxo alcohols, and isocyanates.
There are two main approaches to convert remote natural gas assets into conventional transportation fuels and lubricants using syngas. Natural gas may be converted into syngas followed by a Fischer-Tropsch process, or natural gas may be converted into syngas followed by methanol synthesis, which is followed by a methanol to gas process (MTG) to convert methanol into highly aromatic gasoline. The syngas generation is the most costly step of these processes. A critical feature of these processes is producing syngas with a desired H2/CO ratio to optimize formation of the desired products and to avoid problems in the syngas formation step.
Syngas can be generated from three major chemical reactions. The first involves steam reforming of methane. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 3.0. A second process for syngas generation involves dry reforming of methane or the reaction between carbon dioxide and methane. An attractive feature of this method is that carbon dioxide is converted into syngas; however, this method has problems with rapid carbon deposition. The carbon deposition or coke forming reaction is a separate reaction from the one that generates the syngas and occurs subsequent to the syngas formation reaction. However, the reaction of methane in dry reforming is slow enough that long residence times are required for high conversion rates and these long residence times lead to coke formation. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 1.0. A third process for synthesis gas generation involves partial oxidation of methane using oxygen, where the oxygen can be either air, enriched air, or oxygen with a purity in excess of 90%, preferably in excess of 99%. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 2.0. However, in commercial practice, some amount of steam is typically added to a partial oxidation reformer in order to control carbon formation and the addition of steam tends to increase the H2/CO ratio above 2.0. Likewise it is common to add relatively small amounts of CO2 to the feed gas mixture in an attempt to adjust the ratio closer to 2.0.
Unless otherwise stated, syngas ratios (and percentage compositions) as described herein are in terms of molar ratios (and molar percentages).
It is possible to produce syngas with a H2/CO ratio that is above the ratio ideally desired for the process in which the syngas is to be used, and then to remove excess hydrogen to adjust the ratio to the desired value. However, the H2 removal process employs expensive H2 separation systems that tend to foul and decline in performance with use.
Through the use of a Caloric Calcor process, it is also known to produce high purity carbon monoxide or an oxo-feedstock of hydrogen and carbon monoxide in a ratio of between 0.5 and 1 using a LPG feedstock and carbon dioxide, as described in xe2x80x9cMake CO from CO2,xe2x80x9d Hydrocarbon Processing, Vol. 64, May 1985, pp. 106-107 and xe2x80x9cA new process to make Oxo-feed,xe2x80x9d Hydrocarbon Processing, Vol. 66, July 1987, pg. 52.
The conversion of natural gas to combustible liquid fuels may also involve the production of LPG. A syngas processing facility, such as, for example, a hydrocarbon synthesis facility, typically produces LPG as well as the desired products. The export of LPG from such a facility or from the parent natural gas field is often difficult and expensive. LPG must be compressed and liquefied, and the shipment requires the use of special ocean-going vessels. Furthermore, the market for mixtures of propane and butane is small and of low value. Thus, the LPG must be separated into individual propane and butane of purity to meet specifications for sale. This complicated and expensive operation often results in high costs, which make the value of the LPG at the site of production small.
The conversion of natural gas to combustible liquid fuels further involves the production of some amount greenhouse gas emissions, such as CO2. The production of significant amount of CO2 is environmentally undesirable.
Accordingly, there is a need for a process for producing a syngas with a pre-selected H2/CO ratio that can be varied according to the process in which the syngas is to be employed and that avoids H2 separation and coking in the syngas formation step. There is also a need for a process that minimizes or eliminates production of LPG from a processing facility, such as, for example, a hydrocarbon synthesis facility. Furthermore, there is a need to reduce the greenhouse emissions from a processing facility, such as, for example, a hydrocarbon synthesis facility.
Methods for forming syngas with a variable H2/CO ratio are disclosed. One aspect of the present invention is a process for the production of a blended syngas feed with a desired H2/CO ratio. This process comprises selecting a desired H2/CO ratio of the blended syngas feed. The desired syngas ratio may be in the range of from approximately 1.0 to 3.0. The H2/CO ratio is selected on the basis of the process in which the syngas is to be used. In this process a first syngas is formed with a H2/CO ratio of at least 2.0 by reacting methane with oxygen and water and a second syngas is formed with a H2/CO ratio of no more than 1.5 by reacting LPG (liquified petroleum gas) with CO2. The first syngas and the second syngas are blended to form the blended syngas feed with the desired H2/CO ratio. The blended syngas may be fed to a syngas conversion reactor, and this reactor may be used in a gas to liquid (GTL) conversion process. The blended syngas feed may preferably be a feed for a Fischer-Tropsch reactor, and therefore, the blended syngas may be fed to a Fischer-Tropsch reactor.
An additional aspect of the present invention is a process of using LPG and CO2 in preparing a syngas feed for a Fischer-Tropsch reactor. CO2 and LPG are contacted at reforming reaction conditions to form a first syngas with a H2/CO ratio of not more than 1.5. The first syngas is blended with a second syngas with a H2/CO ratio of no less than 2.0 to form a blended syngas feed. The blended syngas feed is fed into the Fischer-Tropsch reactor.
A further aspect of the present invention is an integrated process for producing a blended syngas with a variable H2/CO ratio for a Fischer-Tropsch reactor. In this process a desired H2/CO ratio of a blended syngas feed to a Fischer-Tropsch reactor is selected. This process comprises reacting methane, oxygen, and steam to form a first syngas with a H2/CO ratio of at least 2.0. A second syngas is formed with a H2/CO ratio of no more than 1.5 by reacting LPG and CO2. The first syngas and the second syngas are blended to form a blended syngas feed for the Fischer-Tropsch reactor having the desired H2/CO ratio. A Fischer-Tropsch synthesis process is performed using the blended syngas feed. Unreacted syngas containing CO2, H2, CO, and CH4 is recovered from the Fischer-Tropsch reactor, and LPG is also recovered from the Fischer-Tropsch reactor.
An additional aspect of the present invention is a process for the production of a blended syngas feed with a variable H2/CO ratio. In this process a first syngas comprising H2 and CO and having a H2/CO ratio in the range of from approximately 1.4 to 1.75 is fed into a first Fischer-Tropsch synthesis reactor and at least one effluent is recovered therefrom. A second syngas comprising H2 and CO is recovered from the effluent wherein the second syngas has a lower H2/CO ratio than that of the first syngas. A third syngas with a H2/CO ratio of at least 2.0 s formed by reacting methane with an oxygen source. The second syngas is blended with the third syngas to form a blended syngas feed having a H2/CO ratio in the range of 1.4 to 1.75. The blended syngas feed is fed into a second Fischer-Tropsch reactor.
In a further aspect the present invention is directed to a process for the production of a blended syngas feed with a variable H2/CO ratio. In this process a first syngas comprising H2 and CO and having a H2 and CO ratio in the range of 1.25 to 2.1 is fed into a first Fischer-Tropsch synthesis reactor and at least one effluent is recovered therefrom. A second syngas is recovered from the effluent wherein the second syngas has a higher H2/CO ratio than that of the first syngas. A third syngas with a H2/CO ratio of no more than 1.5 is formed by reacting LPG with CO2. The second syngas is blended with the third syngas to form a blended syngas feed having a H2/CO ratio in the range of 1.4 to 1.75. The blended syngas feed is fed into a second Fischer-Tropsch reactor.
In an additional aspect, the present invention is directed to a process for producing fuel comprising reacting LPG and CO2 to form a first syngas with a H2/CO ratio of no more than 1.5. The syngas is reacted in a Fischer-Tropsch process to produce a hydrocarbonaceous effluent and at least a portion of the hydrocarbonaceous effluent is converted into at least one fuel.
In a separate embodiment, the present invention is directed to a process for converting gaseous and/or liquid hydrocarbons into C5+ hydrocarbons, wherein the process has a carbon efficiency of greater than 75%. The process may comprise reforming to convert gaseous and/or liquid hydrocarbons into a syngas, hydrocarbon synthesis to convert the syngas to a hydrocarbonaceous product, separating the hydrocarbonaceous product to recover CO2 and C5+ hydrocarbons, and converting at least a portion of the CO2 into C5+ hydrocarbons. In this process a desired H2/CO ratio of a blended syngas feed to a Fischer-Tropsch reactor is selected. This process comprises reacting methane, oxygen, and steam to form a first syngas with a H2/CO ratio of at least 2.0. A second syngas is formed with a H2/CO ratio of no more than 1.5 by reacting LPG and CO2. The first syngas and the second syngas are blended to form a blended syngas feed for the Fischer-Tropsch reactor having the desired H2/CO ratio. A Fischer-Tropsch synthesis process is performed using the blended syngas feed. Unreacted syngas containing CO2, H2, CO, and CH4 is recovered from the Fischer-Tropsch reactor, and LPG is also recovered from the Fischer-Tropsch reactor.